1. Field of the Invention
The present invention is directed to a process for the preparation of aqueous dispersions of blocked polyisocyanates, to the blocked polyisocyanate prepared by this process and to their use in aqueous dispersions which additionally contain an isocyanate-reactive component.
2. Description of the Prior Art
U.S. Pat. No. 4,904,522 is directed to the preparation of water dispersible polyisocyanates for use in fiberglass binder applications. In addition, U.S. Pat. No. 4,663,377 is directed to the use of water dispersible polyisocyanates in adhesive applications.
The disadvantage of these products is that after they are dispersed in aqueous media, they begin to react with water, which means these products have a limited potlife. Waiting too long to apply the product usually results in precipitation and/or formation of insoluble, non-film forming ureas. Even when these products are applied directly after the aqueous dispersions are formed, there is a problem. If film formation occurs before the water/isocyanate reaction is complete, subsequent carbon dioxide formation will result in bubbles in the film rendering it useless for coatings applications.
One solution to the problems of limited potlife and bubble formation is to block the isocyanate groups of the water dispersible polyisocyanate prior to dispersing it in water as described in U.S. Pat. Nos. 4,522,851, 4,098,933 and 4,284,544. The disadvantage of blocking the polyisocyanate before it is dispersed in water is that the blocked polyisocyanate is generally a solid or a high viscosity liquid which is very difficult or impossible to disperse in water without first diluting the blocked polyisocyanate in an organic solvent or heating the blocked polyisocyanate to an elevated temperature to reduce its viscosity.
Because one of the primary reasons for dispersing polyurethanes and polyisocyanates in water is to avoid the unnecessary use of organic solvents, it is certainly not desirable to have to dilute the blocked polyisocyanate with an organic solvent in order to reduce its viscosity sufficiently to disperse it in water. In addition, the extra energy costs and time which are necessary to heat the blocked polyisocyanate to an elevated temperature to reduce its viscosity are also undesirable.
Accordingly, an object of the present invention is to provide a process for preparing a blocked polyisocyanate which does not require substantial amounts of solvent or elevated temperatures in order to disperse the polyisocyanate in water.
Surprisingly, this object may be achieved in accordance with the present invention as described hereinafter.